Sumner, let's see how to use variables with functions in Mathcad.
Let's work with this function called g(x).
To define a function, type the function name and then use the assignment operator.
Now we'll type the rest of the function.
Let's evaluate it for x = 1 and x = 3.
We can also evaluate the function over a range of values.
Now we want to graph the function. Click on the graph button in the symbol palette.
The graph appears with placeholders. We'll plot this graph with the value for x along the x axis...
...and the function name on the y axis.
What if I want a bar graph instead?
You can change the graph style by clicking on the graph and choosing Format from the X-Y Graph menu.
From here you can change all the characteristics of your graph.
To change from lines to bars, click on Trace 1. A trace is the graphed line.
Click on the arrow under the Trace type column. Select bar from the drop down list and click OK.
There's your bar graph!
We can also graph one of Mathcad's over 100 built-in functions.
First we'll enter the range for x.
Then, click on the graph button.
We'll plot the function along the y axis over the range of values x on the x axis.
Just type in the function, and Mathcad recognizes it. Let's type in the function sine of x.
And there's the sine wave!
Can I plot more than one function on the same graph?
Sure! Let's try plotting cosine of x on the same graph. 
Place the cursor after the first function on the y axis, press the spacebar to select the expression, and type a comma.
Notice that a new placeholder appears for you to type in your next function.
So, Mathcad can display more than one function on a graph.